Your question is inaccurate, as to imply that he does not believe in Christ. Which he does. Many people think the Bible is a collection of stories that are divinely inspired. We cannot know all the facts, but archaeological research has found much evidence to support many things in the Bible. Here is a brief article about the interview with Bush:

Bush: Bible, evolution not at odds
Dec 8, 2008
WASHINGTON (AFP) — US President George W. Bush said in an interview Monday that the Bible is "probably not" literally true and that a belief that God created the world is compatible with the theory of evolution.
"I think you can have both," Bush, who leaves office January 20, told ABC television, adding "You're getting me way out of my lane here. I'm just a simple president."
But "evolution is an interesting subject. I happen to believe that evolution doesn't fully explain the mystery of life," said the president, an outspoken Christian who often invokes God in his speeches.
"I think that God created the Earth, created the world; I think the creation of the world is so mysterious it requires something as large as an almighty and I don't think it's incompatible with the scientific proof that there is evolution," he told ABC television.
Asked whether the Bible was literally true, Bush replied: "Probably n...'""''"

Your question is inaccurate, as to imply that he does not believe in Christ. Which he does. Many people think the Bible is a collection of stories that are divinely inspired. We cannot know all the facts, but archaeological research has found much evidence to support many things in the Bible. Here is a brief article about the interview with Bush:

Bush: Bible, evolution not at odds
Dec 8, 2008
WASHINGTON (AFP) — US President George W. Bush said in an interview Monday that the Bible is "probably not" literally true and that a belief that God created the world is compatible with the theory of evolution.
"I think you can have both," Bush, who leaves office January 20, told ABC television, adding "You're getting me way out of my lane here. I'm just a simple president."
But "evolution is an interesting subject. I happen to believe that evolution doesn't fully explain the mystery of life," said the president, an outspoken Christian who often invokes God in his speeches.
"I think that God created the Earth, created the world; I think the creation of the world is so mysterious it requires something as large as an almighty and I don't think it's incompatible with the scientific proof that there is evolution," he told ABC television.
Asked whether the Bible was literally true, Bush replied: "Probably not. No, I'm not a literalist, but I think you can learn a lot from it."
"The important lesson is 'God sent a son,'" he said.